The present invention relates generally to liquid-atomizing spray nozzles, and more particularly to an air-assisted or xe2x80x9cairblastxe2x80x9d fuel nozzle for turbine combustion engines, the nozzle having a multiplicity of aerodynamic turning vanes arranged to define an outer air xe2x80x9cswirlerxe2x80x9d providing for a more uniform atomization of the fuel flow stream.
Liquid atomizing nozzles are employed, for example, in gas turbine combustion engines and the like for injecting a metered amount of fuel from a manifold into a combustion chamber of the engine as an atomized spray of droplets for mixing with combustion air. The fuel is supplied at a relatively high pressure from the manifold into, typically, an internal swirl chamber of the nozzle which imparts a generally helical component vector to the fuel flow. The fuel flow exits the swirl chamber and is issued through a discharge orifice of the nozzle as a swirling, thin, annular sheet of fuel surrounding a central core of air. As the swirling sheet advances away from the discharge orifice, it is separated into a generally-conical spray of droplets, although in some nozzles the fuel sheet is separated without swirling.
In basic construction, fuel nozzle assemblies of the type herein involved are constructed as having an inlet fitting which is configured for attachment to the manifold of the engine, and a nozzle or tip which is disposed within the combustion chamber of the engine as having one or more discharge orifices for atomizing the fuel. A generally tubular stem or strut is provided to extend in fluid communication between the nozzle and the fitting for supporting the nozzle relative to the manifold. The stem may include one or more internal fuel conduits for supplying fuel to one or more spray orifices defined within the nozzle. A flange may be formed integrally with the stem as including a plurality of apertures for the mounting of the nozzle to the wall of the combustion chamber. Appropriate check valves and flow dividers may be incorporated within the nozzle or stem for regulating the flow of fuel through the nozzle. A heat shield assembly such as a metal sleeve, shroud, or the like additionally is included to surround the portion of the stem which is disposed within the engine casing. The shield provides a thermal barrier which insulates the fuel from carbonization or xe2x80x9cchoking,xe2x80x9d the products of which are known to accumulate within the orifices and fuels passages of the nozzle and stem resulting in the restriction of the flow of fuel therethrough.
Fuel nozzles are designed to provide optimum fuel atomization and flow characteristics under the various operating conditions of the engine. Conventional nozzle types include simplex or single orifice, duplex or dual orifice, and variable port designs of varying complexity and performance. Representative nozzles of these types are disclosed, for example, in U.S. Pat. Nos. 3,013,732; 3,024,045; 3,029,029; 3,159,971; 3,201,050; 3,638,865; 3,675,853; 3,685,741; 3,899,884; 4,134,606; 4,258,544; 4,425,755; 4,600,151; 4,613,079; 4,701,124; 4,735,044; 4,854,127; 4,977,740; 5,062,792; 5,174,504; 5,269,468; 5,228,283; 5,423,178; 5,435,884; 5,484,107; 5,570,580; 5,615,555; 5,622,054; 5,673,552; and 5,740,967.
As issued from the nozzle orifice, the swirling fluid sheet atomizes naturally due to high velocity interaction with the ambient combustion air and to inherent instabilities in the fluid dynamics of the vortex flow. However, the above-described simplex or duplex nozzles also may be used in conjunction with a stream of high velocity and/or high pressure air, which may be swirling, applied to one or both sides of the fluid sheet. In certain applications, the air stream may improve the atomization of the fuel for improved performance. Depending upon whether the air is supplied from a source external or internal to the engine, these xe2x80x9cair-atomizingxe2x80x9d nozzles which employ an atomization air stream are termed xe2x80x9cair-assistedxe2x80x9d or xe2x80x9cairblast.xe2x80x9d Airblast and air-assisted nozzles have been described as having an advantage over what arc termed xe2x80x9cpressurexe2x80x9d atomizers in that the distribution of the fluid droplets through the combustion zone is dictated by a airflow pattern which remains fairly constant over most operations conditions of the engine. Nozzles of the airblast or air-assisted type are described further in U.S. Pat. Nos. 3,474,970; 3,866,413; 3,912,164; 3,979,069; 3,980,233; 4,139,157; 4,168,803; 4,365,753; 4,941,617; 5,078,324; 5,605,287; 5,697,443; 5,761,907; and 5,782,626.
Most, if not all, of the aforementioned nozzle designs incorporate swirlers or other turning vanes to impart a generally helical motion to one or more of the fluid flow streams within the nozzle. For example, certain airblast nozzles employ an outer air swirler configured on the surface of a generally-annular member which forms the primary body of the nozzle. In this regard, the body has an inlet orifice and outlet orifice or discharge for the flow of inner air and fuel streams. A series of spaced-apart, parallel turning vanes are provided on a radial outer surface of the body as disposed circumferentially about the discharge orifice. As incorporated into the nozzle, the primary nozzle body is coaxially disposed within a surrounding, secondary nozzle body or shroud such that the radial outer surface of the primary nozzle body defines an annular conduit with a concentric inner surface of the secondary nozzle body for the flow of an outer, atomizing air stream. As each of the vanes is disposed at an angle relative to the central longitudinal axis of the swirler and the direction of air flow, a helical motion is imparted to the atomizing air which exits the nozzle as a swirling stream.
Particularly with respect to airblast or air-assisted nozzles of the type herein involved, the ability to produce a desired fuel spray which is finely atomized into droplets of uniform size is dependent upon the preparation of the atomizing air flow upstream of the atomization point. That is, excessive pressure drop or other loss of velocity in the atomization air can result in larger droplets and a coarser fuel spray. Large or non-uniform droplets also can result from a non-uniform velocity profile or other gradients such as wakes and eddies in the atomizing air flow.
Heretofore, air swirlers of the type herein involved have employed vanes of relatively simple slots or flats, or helical or curved geometries to guide and control fluid flow. In certain applications, however, slots or vanes of these types may provide less than optimum performnance. In this regard, reference may be had to FIG. 1 wherein fluid flow through a pair of parallel, helical vanes is shown in schematic at 10. Each of the helical vanes, referenced at 12a and 12b, has a leading edge, 14a-b, and a trailing edge, 16a-b, respectively, and is disposed at a turning or incidence angle, xcex8, relative to the upstream direction of fluid flow which is indicated by arrow 18. The vanes are spaced-apart radially to define a flow passage, referenced at 20, therebetween.
As may be seen in the schematic of FIG. 1, with the fluid flow being directed to define a lower pressure or suction side, referenced at xe2x80x9cS,xe2x80x9d and a higher pressure or pressure side, referenced at xe2x80x9cP,xe2x80x9d of the vanes 12, some separation of the flow from the suction side is evident beginning at the leading edge 14 of each of the vanes. This separation, which produces the leading edge bubbles depicted by the streamlines referenced at 22a-b, and the trailing edge wakes, eddies, vorticities, or other recirculation flow depicted by the streamlines referenced at 24a-b, has the effect of reducing the area for fluid flow through the vane passages 20, and of developing strong secondary flows within the stream which can persist many vane lengths downstream of the vanes 12. Thus, and particularly for medium or high turning angles, i.e., between about greater than about 8xc2x0, a helical vane profile can result in a diminished flow volume from the nozzle, non-uniform downstream velocity profiles, and otherwise in velocity or pressure losses and than optimum performance.
Turning next to FIG. 2, the fluid flow through a pair of parallel, curved vanes is shown for purposes of comparison at 10xe2x80x2. As before, each of the curved vanes 12a-bxe2x80x2 has a leading edge 14a-bxe2x80x2, and a trailing edge 16a-bxe2x80x2, respectively, and is disposed at a turning or incidence angle, xcex8, relative to the direction of fluid flow which again is indicated by arrow 18. The vanes are spaced-apart radially to define a flow passage 20xe2x80x2 therebetween.
As compared to that of the helical vanes of FIG. 1, the flow through the curved vanes 12xe2x80x2 exhibits no appreciable bubble separation at the leading edges 14. However, as the trailing edges 16xe2x80x2 of the vanes are not parallel, that is the suction side S of vane 12axe2x80x2 is not parallel to the pressure side P of vane 12bxe2x80x2, losses are produced and the flow becomes non-uniform at that point as shown by the separation referenced at 24a-bxe2x80x2. At large turning angles, i.e., greater than about 15xc2x0, the effect becomes more pronounced and may result in pressure losses, non-uniform velocity profiles, and recirculation flows downstream.
In view of the foregoing, it will be appreciated that improvements in the design of fuel nozzles for turbine combustion engines and the like would be well-received by industry. A preferred design would ensure a uniform atomization profile under a range of operating conditions of the engine.
The present invention is directed principally to airblast or air-assisted fuel nozzles for dispensing an atomized fluid spray into the combustion chamber of a gas turbine engine or the like, and particularly to an outer air swirler arrangement for such nozzles having an aerodynamic vane design which minimizes non-uniformities, such as separation, pressure drop, azimuthal velocity gradients, and secondary flows in the atomizing air flow. The swirler arrangement of the present invention thereby produces a relatively uniform, regular flow downstream of the vanes which minimizes entropy generation and energy losses and maximizes the volume or mass flow rate of air through the vane passages. Without being bound by theory, it is believed that, as the velocity and total pressure of the swirling atomizing air as it impinges the annular liquid sheet is substantially uniform, the formation of large droplets in the atomized sheet is minimized. Moreover, as the velocity of the atomizing air is higher due to reduced total pressure losses, the formation of small droplets is believed to be facilitated. The overall result is that the atomization performance of a given nozzle may be enhanced to provide a smaller mean droplet size over the full range of turning angles typically specified for turbine combustion engines. Equivalently, less atomization air is required to achieve a specified droplet size.
As the name implies, the xe2x80x9caerodynamicxe2x80x9d vanes of the present invention are characterized as having the general shape of an airfoil with a leading edging and a trailing edge, and are arranged radially about the outer circumference of the swirler such that the trailing edge surfaces of adjacent vanes are generally parallel. As is shown in U.S. Pat. Nos. 5,588,824; 5,351,477; 5,511,375; 5,394,688; 5,299,909; 5,251,447; 4,246,757; and 2,526,410, aerodynamic vanes have been utilized for turbine blades, and within the nozzle or combustion chamber to direct the flow of combustion air. Heretofore, however, it was not appreciated that such vanes also might be used to guide the flow of atomizing air in airblast nozzles. Indeed, it was not expected that the atomization performance of existing airblast nozzles could be rather dramatically improved while still satisfying such constraints as structural integrity, envelope size, and manufacturability at a reasonable cost.
In an illustrated embodiment, the air-atomizing fuel nozzle of the invention is provided as including a body assembly with an inner fuel passage and an annular outer atomizing air passage. The inner fuel passage extends axially along a longitudinal axis to a first terminal end defining a first discharge orifice of the nozzle. The outer atomizing air passage extends coaxially with the inner fuel passage along the longitudinal axis to a second terminal end disposed concentrically with the first terminal end and defining a second discharge orifice oriented such that the discharge therefrom impinges on the fuel discharge from the first discharge orifice. An array of turning vanes is disposed within the outer atomizing air passage in a circular locus about the longitudinal axis. Each of the vanes is configured generally in the shape of an airfoil and has a pressure side and an opposing suction side. The vanes extend axially from a leading edge surface to a tapering trailing edge surface along a corresponding array of chordal axes, each of which axes is disposed at a given turning angle to the longitudinal axis. The suction side of each vane is spaced-apart from a juxtaposing pressure side of an adjacent vane to define a corresponding one of a plurality of aligned air flow channels therebetween.
In operation, a fuel flow is directed through the inner fuel passage with atomizing air flow being directed through the flow channels of the outer air passage. Fuel is discharged into the combustion chamber of the engine from the first discharge orifice and as a generally annular sheet, with atomizing air being discharged from the second discharge orifice flow as a surrounding swirl which impinges on the fuel sheet. As a result of the uniform velocity profile developed in the swirl by the effect of the aerodynamic turning vanes, the sheet is atomized into a spray of droplets of more uniform size.
The present invention, accordingly, comprises the apparatus and method possessing the construction, combination of elements, and arrangement of parts and steps which are exemplified in the detailed disclosure to follow. Advantages of the present invention include an airblast or air-assisted nozzle construction which provides for a reduction in the mean droplet size in the liquid spray, and which utilizes less atomizing air to effect a specified droplet size. Additional advantages include an airblast or air-assisted nozzle which provides consistent atomization over a full range of turning angles and a wide range of engine operating conditions.
These and other advantages will be readily apparent to those skilled in the art based upon the disclosure contained herein.